End to End Encryption Between Users

*This issue was originally created by [**@zin1x**](https://github.com/zin1x) at .*

### Background information

- **Dendrite version or git SHA**: 0.5.1+a47b12dc
- **Monolith or Polylith?**: Monolith
- **SQLite3 or Postgres?**: Postgres
- **Running in Docker?**: No
- **`go version`**: go version go1.17.3 linux/amd64

### Description

End-to-end encryption of DM between 2 user does not work the moment user logs off

### Steps to reproduce

- User 1 invite User 2 to a chat
- Conversation exchanges are encrypted
- User 2 log off
- User 2 log on
- User 2 message User 1 in the previously established chat
- on Element.io/Web - message is sent *unencrypted* to User 2
- on Element App - message from User 2 is *will never be unable to send* to User 1

Start by reproducing the reported flow in a two-user encrypted DM: establish encryption, log User 2 out and back in, then send a message. Investigate how Dendrite handles the encrypted session across logout and login; done means messages remain encrypted and can be sent and received successfully in both Element Web and the Element app.

Written by the indexing model from the issue text.